1. Field of the Invention
The present invention relates to an exhaust gas purification device for an internal combustion engine which operates at a lean air-fuel ratio, for example, an engine such as a diesel engine or a lean-burn gasoline engine. More specifically, the present invention relates to an exhaust gas purification device provided with an NO.sub.x purifying catalyst capable of reducing NO.sub.x in an exhaust gas having a lean air-fuel ratio.
2. Description of the Related Art
An exhaust gas purification device utilizing an NO.sub.x purifying catalyst such as a selective reduction catalyst disposed in the exhaust gas passage of an internal combustion engine is known in the art. The selective reduction catalyst is a catalyst which is capable of reducing NO.sub.x in the exhaust gas even though the air-fuel ratio of the exhaust gas is lean, by selectively reacting NO.sub.x with substances such as HC (hydrocarbons) and CO (carbon monoxide) in the exhaust gas.
This type of exhaust gas purification device is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 63-283727. The device in the '727 publication is provided with a selective reduction catalyst disposed in the exhaust gas passage of an internal combustion engine and reduces NO.sub.x in the exhaust gas, by the selective reduction catalyst, by reacting NO.sub.x with HC and CO contained in the exhaust gas of the engine.
When a selective reduction catalyst is used for purifying NO.sub.x in an exhaust gas, it is necessary to supply the substances which can react with and reduce NO.sub.x, such as HC and CO. Usually, HC and CO are fed to the selective reduction catalyst by generating HC and CO in the combustion chamber of the engine, or by supplying a reducing agent to the exhaust gas in the exhaust gas upstream of the selective reduction catalyst (in this specification, a reducing agent means a substance which generates a reducing substances such as CO and H.sub.2, or hydrocarbons HC in the exhaust gas). However, problems arise when the selective reduction catalyst is used for purifying NO.sub.x in a lean air-fuel ratio exhaust gas. It is known that the selective reduction catalyst usually also acts an oxidizing catalyst which oxidizes HC, CO in a lean air-fuel ratio exhaust gas. Therefore, when a reducing agent is supplied to the lean air-fuel ratio exhaust gas flowing into the selective reduction catalyst, almost all of HC and CO in the exhaust gas are oxidized at the portion of the catalyst near the inlet end face thereof and substantially no HC and CO reach the portion near the outlet end of the catalyst. Especially, when a platinum-zeolite catalyst is used as a selective reduction catalyst, substantially no HC, CO supplied to the catalyst reaches the latter half of the catalyst since the oxidizing ability of platinum (Pt) is large. In this case, since NO.sub.x is not reduced in the latter half of the catalyst due to the shortage of HC, CO, the NO.sub.x purification ability of the device as a whole becomes low.
Further, usually a small amount of SO.sub.2 (sulfur dioxide) generated by the combustion of sulfur in fuel is contained in the exhaust gas of the engine. When SO.sub.2 contacts an oxidizing catalyst in an oxidizing atmosphere, sulfate such as SO.sub.3 (sulfur trioxide) is formed by the oxidation of SO.sub.2. SO.sub.3 further forms H.sub.2 SO.sub.4 (sulfuric acid mist) by reacting with H.sub.2 O in the catalyst or in the exhaust gas. Since sulfuric acid mist in the exhaust gas is detected as particulates, the amount of the particulate in the exhaust gas increases when an oxidizing catalyst is used.
Therefore, when a selective reduction catalyst which also acts as an oxidizing catalyst in a lean air-fuel ratio exhaust gas is used for purifying NO.sub.x, a problem occurs in that the amount of the particulates in the exhaust gas increases due to the formation of sulfate (hereinafter, it should be understood that the term "sulfate" means both SO.sub.3 and H.sub.2 SO.sub.4).
It is also known that sulfate is reduced to SO.sub.2 on the selective reduction catalyst by reacting with reducing agent. Therefore, if more than two selective reduction catalysts are arranged in the exhaust gas passage of the engine in series, sulfate formed by the upstream catalyst may be reduced to SO.sub.2 by the downstream catalyst. However, also in this case, since the reducing agent supplied to the exhaust gas upstream of the upstream catalyst is oxidized by the upstream catalyst, substantially no reducing agent reaches the downstream catalyst. Therefore, sulfates in the exhaust gas are not reduced on the downstream selective reduction catalyst. Thus, on the downstream catalyst, an increase in the amount of particulates and an insufficient reduction of NO.sub.x in the exhaust gas occur.
The problems set forth above may be solved if a plurality of selective reduction catalysts are arranged in the exhaust gas passage in series, and if the reducing agent is supplied to the exhaust gas at the inlets of the respective catalysts. However, in this case, the device for supplying the reducing agent is required for each of the catalysts and the problems of a complication of the system and an increase in the manufacturing cost arise.